In recent years, from a perspective of preserving the environment, it is becoming popular to use a power generating apparatus of renewable energy type such as a wind turbine generator utilizing wind power and renewable energy type turbine generator such as a tidal current generator utilizing tidal. In the power generating apparatus of renewable energy type, the kinetic energy of the wind, tidal current, river current or the like is converted to the rotation energy of the rotor and the rotation energy of the rotor is then converted into electric power by the generator.
In this type of conventional power generating apparatus of renewable energy type, a rotation speed of the rotor is small compared to the rated rotation speed of the generator and thus, a mechanical gearbox is provided between the rotor and the generator to increase the rotation speed of the rotor to the rated rotation speed of the generator and then input the increased rotation speed to the generator.
However, as the power generating apparatus of renewable energy type increases in size for the purpose of enhancing the power generation efficiency, weight and cost of the gearbox tends to increase as well. Thus, a power generating apparatus equipped with the hydraulic transmission having the hydraulic pump and the hydraulic motor, instead of the mechanical gearbox, is becoming popular.
For instance, in Patent Literature 1, a power production system transmits rotation energy of a rotor to a generator via a hydraulic transmission. This system is configured such that a hydraulic motor and the generator are provided in a nacelle (see FIG. 7 of Patent Literature 1).
Further, in Patent Literature 1, a wind turbine generator is configured such that a hydraulic motor arranged at a bottom of a tower turns with a nacelle around a vertical axis.
Furthermore, in Patent Literature 3 and Patent Literature 4, disclosed is a wind turbine generator in which a part of a hydraulic piping (a high pressure oil path and a low pressure oil path) turns with a nacelle, the hydraulic piping connecting a hydraulic pump arranged in the nacelle and a hydraulic motor arranged at a bottom of a tower. In this wind turbine generator, a hydraulic swivel arranged at a bottom of the nacelle allows the hydraulic piping on the nacelle side to turn with the nacelle. The hydraulic swivel consists of an outer element and an inner element which are rotatable relative to each other. The piping arranged on the inner element is in communication with an annular path provided in an inner periphery of the outer element.